High speed data links can use optical fibers to communicate optical data signals over significant distances with low signal loss. Single mode optical fibers having losses of less than 0.20 dB/km at wavelengths near 1550 nm are typical, and optical transmitters and receivers are readily available that can be simply coupled to optical fibers. Such low losses have led to the implementation of optical fiber based communication systems in data center communications.
To take advantage of the high bandwidths available with optical fiber links, so-called coherent communication systems have been developed that encode data onto amplitude, phase and polarization of an optical beam in a dense wavelength division multiplexing (DWDM) technique and achieve per wavelength data rates of 10 Gbit/s and higher. Such systems typically use dual polarization-quadrature phase shift keying (DP-QPSK) modulation and coherent optical detection. Higher order quadrature amplitude modulation (QAM) formats have also been used with coherent detection.
Other approaches for high speed data communication on optical fibers use direct detection. One problem in direct-detection systems for use in DWDM is fiber chromatic dispersion (CD) and fiber nonlinearities that can cause interference and channel degradation and limit communication distances. As data rates increase, both signal bandwidth and modulation format can produce increased optical signal bandwidth, increasing the likelihood of signal degradation due to CD. Multi-level pulse-amplitude-modulation (PAM) formats have been proposed for high data rate data center applications. In such PAM modulation, optical power is modulated to multiple levels with each level representing multiple bits. These systems can be cost-effective and are promising candidates for metro-reach applications. However, in multi-level PAM systems, signal levels are typically much closer to each other than in simple on-off keying (OOK) systems, and are significantly more prone to errors produced by CD and fiber nonlinearities. Even though some tunable CD compensators are available for fiber CD compensation on DWDM channels, conventional tunable CD compensators typically exhibit multiple deficiencies: (i) CD inaccuracies remain over all channels as CD cannot be individually compensated on a per-channel basis, (ii) fixed channel frequency grids are required, limiting the channel plan, and (iii) limited CD compensation resulting in communication bandwidth limitations.